1. Field of the Invention
The present invention relates to an information reading apparatus for optically reading information from an optical pattern such as a bar code which is a printed code representing predetermined information.
2. Description of the Related Art
As an example of a label attached to an article for the purpose of identification, there is known a label with a bar code pattern comprising a series of bars and spaces with various widths, instead of characters and numerals. The bar code represents various information relating to the kind of articles and manufacturing data.
In a conventional information reading apparatus, an infrared beam, a laser beam or the like is radiated on a bar code and a difference in light reflection characteristics resulting from the difference in width between bars and spaces is detected. Thus, a bar code pattern is read and the original information is reproduced, thereby identifying the label or article.
Jap. Pat. Appln. KOKAI Publication No. 2-17888 discloses an example of such a conventional information reading apparatus. The disclosed apparatus comprises a handle unit and a head unit in a handy structure. Even if this information reading apparatus is not closely attached to a bar code, it can read the bar code by radiating a laser beam to a desired label at a distance of, e.g. several meters.
The head unit comprises a laser beam source, a scanning mirror for reflecting a laser beam emitted from the laser beam source, and a concave condensing mirror, formed integral with the scanning mirror, for converging an incident light beam at a photosensor. The scanning mirror is oscillated by a small-sized motor, like a pendulum.
In the above structure, the head unit is directed to the bar code and then a laser beam is emitted from the laser beam source. The laser beam is reflected by the oscillating scanning mirror and directed towards the bar code as a scanning beam.
In this case, since the scanning beam, i.e. the emitted laser beam, is reflected by the oscillating scanning mirror by the small-sized motor, the beam scans over the bar code.
The beam reflected by the bar code is converged at the photosensor by the concave condensing mirror within the head unit. After the beam is converted to an electric signal corresponding to a variation in optical characteristics of the beam, the electric signal is read by a signal processing circuit (not shown) as bar code information.
The above-described conventional information reading apparatus, however, has the following drawbacks.
Suppose, for example, that the laser beam source is positioned below a reference horizontal plane (i.e. an emission angle of a laser beam) and the scanning mirror is positioned such that the center of the scanning mirror overlaps the reference horizontal plane. In this case, a laser beam is emitted from the laser beam source at a predetermined angle to the reference horizontal plane and, after reflected by the scanning mirror, travels along the reference horizontal plane.
Because of the space for ensuring the emission angle of the laser beam, the thickness of the head unit increases by a degree corresponding to the emission angle and it is difficult to reduce the head unit in thickness and size. In addition, since both the laser beam source and photosensor are situated outside the optical path of the reflection beam from the bar code, it is also difficult to reduce the width of the apparatus.
The rotational shaft of the scanning mirror is not situated perpendicular to the optical axis of the laser beam reflected by the scanning mirror. Consequently, as shown in FIGS. 12A and 12B, the scanning beam is scanned over a bar code 7 along a curved locus. If the bar code 7 is located at a far distance, the beam cannot be scanned over all bars of the bar code 7. The defective scanning results in defective information reading.
The conventional information reading apparatus requires the space for installing a small-sized motor for oscillating the scanning mirror and a motor driving circuit. Even if the arrangement of the laser beam source and scanning mirror is changed to partially reduce the size of the reading apparatus, the size and weight of the entire apparatus cannot sufficiently be reduced owing to the presence of the space for the motor and motor driving circuit.
It is desirable to linearly scan the information reading beam (laser beam) at a constant speed in the direction of arrangement of the bars of the bar code, thereby reading the bar code. In order to stably oscillate the scanning mirror, a small-sized motor with high performance is required and therefore the manufacturing cost increases.
Furthermore, there is an idea that the scanning mirror is oscillated by using a resilient member such as a spring in lieu of the small-sized motor. In this case, however, even if the necessary scanning is completed, the oscillation of the resilient member does not stop immediately. If the next scanning operation is started before the oscillation stops, the scanning becomes unstable at the beginning and the bar code may be erroneously read. Therefore, it is necessary to stop the oscillation by some means each time the scanning operation is completed.